The present invention relates generally to an electrical switching device, and more particularly, to a method and apparatus of asynchronously controlling contactors to reduce mechanical stresses on an induction motor during transition between modes of operation.
Typically, contactors are used in starter applications to switch on/off a load as well as to protect a load, such as a motor, or other electrical devices from current overloading. As such, a typical contactor will have three contact assemblies; a contact assembly for each phase or pole of a three-phase electrical device. Each contact assembly typically includes a pair of stationary contacts and a moveable contact. One stationary contact will be a line side contact and the other stationary contact will be a load side contact. The moveable contact is controlled by an actuating assembly comprising an armature and magnet assembly which is energized by a coil to move the moveable contact to form a bridge between the stationary contacts. When the moveable contact is engaged with both stationary contacts, current is allowed to travel from the power source or line to the load or electrical device. When the moveable contact is separated from the stationary contacts, an open circuit is created and the line and load are electrically isolated from one another.
Generally, a single coil is used to operate a common carrier for all three contact assemblies. As a result, the contactor is constructed such that whenever a fault condition or switch open command is received in any one pole or phase of the three-phase input, all the contact assemblies of the contactor are opened in unison. Similarly, when a closed circuit or conducting condition is desired, all the contact assemblies are controlled to close in unison. Simply, the contact assemblies are controlled as a group as opposed to being independently controlled.
This contactor construction has some drawbacks, particularly in high power motor starter applications. Since there is a contact assembly for each phase of the three-phase input, the contact elements of the contact assembly must be able to withstand high current conditions or risk being weld together under fault (high current) or abnormal switching conditions. The contacts must therefore be fabricated from composite materials that resist welding. These composite materials can be expensive and contribute to increased manufacturing costs of the contactor. Other contactors have been designed with complex biasing mechanisms to regulate “blow open” of the contacts under variable fault conditions, but the biasing mechanisms also add to the complexity and cost of the contactor. Alternately, to improve contact element resistance to welding without implementation of more costly composites can require larger contact elements. Larger contacts provide greater heat sinking and current carrying capacity. Increasing the size of the contact elements, however, requires larger actuating mechanisms, coils, biasing springs, and the like, which all lead to increased product size and increased manufacturing costs.
Additionally, a contactor wherein all the contact assemblies open in unison can result in contact erosion as a result of arcs forming between the contacts during breaking. When all the contact assemblies or sets of contacts are controlled in unison, a detected abnormal condition, such as a fault condition, in any phase of the three-phase input causes all the contact assemblies to break open because the contact assemblies share a bridge or crossbar. Therefore, breaking open of the contacts of one contact assembly causes the contacts of the other contact assemblies to also open. As a result, the contacts may open at non-ideal current conditions. For example, the contactor may be controlled such that a fault condition is detected in the first phase of the three phase input and the contacts of the corresponding assembly are controlled to open when the current in the first phase is at a zero crossing. Since the second and third phases of a three phase input lag the first phase by 120 and 240 degrees, respectively, breaking open of the contacts for the contact assemblies for the second and third phases at the opening of the contacts of the contact assembly of the first phase causes the second and third contact assemblies to open when the current through the contacts is not zero. This non-zero opening can cause arcing between the contact elements of the second and third contact assemblies causing contact erosion that can lead to premature failure of the contactor. This holds true for both abnormal switching as stated above as well as normal duty.
This unison-controlled construction also has disadvantages associated with the closing of contacts to cause high transient current conduction between a power source and a load. The closing in unison of all the contacts can cause mechanical torque oscillations that are often negative. As a result, the windings of a motor as well as the mechanical components of the system are subjected to damaging stresses. Additionally, the motor circuit protection disconnect (breaker or fusing) may have to be oversized to avoid unwanted, or “nuisance”, tripping by this high transient current. This is particularly problematic for motor starting applications.
A common technique for starting a three-phase induction motor involves the simultaneous application of full voltage to all three windings of the motor. This technique is generally referred to as Direct on Line (DOL) switching. Generally, a three-phase electromagnetic contactor assembly is used to control the application of voltage to the motor windings. At start-up of the motor, the three sets of contacts of the contactor assembly are closed simultaneously to apply full voltage to all three windings of the motor. For motors with six terminals (two terminals accessible for each stator winding) the contacts are connected so that upon closure, all three windings are energized simultaneously. Heretofore, this simultaneous closure has been achieved with an electromagnetic contactor consistent with that described above where the three sets of contacts share a common actuating assembly.
The drawbacks of such an in-unison contactor assembly design can be particularly damaging for motor starting applications. That is, the build up of torque in the motor generally associated with DOL starting is accompanied by a strong initial transient torque pulsation, as a result of the sudden application of voltage and current. The torque may oscillate between both positive and negative values, and the swing may be many times the normal full load torque of the motor. The effect of this pulsation is to place high mechanical stress on the motor and the whole drive train—the shaft coupling, the shaft itself, any gears driven by the shaft, and the load being driven. The stator windings also experience an equal and opposite reaction to the forces generated.
Moreover, if all the poles of a motor starter contactor assembly are closed simultaneously, power is applied simultaneously to all three windings of the induction motor that can inject a DC transient current in addition to the AC current to the motor. This superposition of the DC and AC currents can then cause high inrush currents, current imbalances, and strong torque pulsations until the DC current gradually decays with the magnetization time constant of the motor.
The DC transient resulting from the simultaneous application of power to the three windings of an induction motor will now be set forth mathematically. The three-phase supply voltage may be described by a space vector ūs(t) given by:ūS(t)=uSej(wt+α)  (Eqn. 1),where us is the supply phase voltage amplitude, and the space vector ūS(t) rotates at the angular frequency ω of the supply; and α is the supply phase angle at the time t=0 when power is applied.
The build up of flux ψ in the motor according to Faraday's Law is given by:
                                          ⅆ                          ψ              _                                            ⅆ            t                          =                                                            u                _                            s                        ⁡                          (              t              )                                =                                    u              s                        ⁢                          ⅇ                              j                ⁢                                                                  ⁢                wt                                      ⁢                                          ⅇ                α                            .                                                          (                  Eqn          .                                          ⁢          2                )            Integration therefore yields:
                                                        ψ              _                        ⁡                          (              t              )                                =                                                    u                s                            ⁢                                                ⅇ                                      j                    ⁢                                                                                  ⁢                    wt                                                                    j                  ⁢                                                                          ⁢                  ω                                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                  α                                                      +                                          ψ                _                            DC                                      ,                            (                  Eqn          .                                          ⁢          3                )                                                          ⁢                              =                                                                                ψ                    _                                    ⁡                                      (                    t                    )                                                                    Steady                  ⁢                                                                          ⁢                  state                                            +                                                ψ                  _                                                  DC                  ⁢                                                                          ⁢                  transient                                                              ,                                    (                  Eqn          .                                          ⁢          4                )            where ψDC transient is the constant of integration required to satisfy initial conditions. When ūS(t) is applied to the motor at t=0 and at phase angle α with no flux in the motor (i.e. ψ=0), then:
                                          ψ            _                    ⁡                      (            0            )                          =                  0          =                                                    u                s                            ⁢                                                ⅇ                                      j                    ⁢                                                                                  ⁢                    α                                                                    j                  ⁢                                                                          ⁢                  ω                                                      +                                                            ψ                  _                                DC                            .                                                          (                  Eqn          .                                          ⁢          5                )            Hence, the DC transient flux can be given by:
                                                        ψ              _                        DC                    =                                                    -                                  u                  s                                            ⁢                                                ⅇ                                      j                    ⁢                                                                                  ⁢                    α                                                                    j                  ⁢                                                                          ⁢                  ω                                                      =                          j              ⁢                                                          ⁢                                                                                          u                      _                                        s                                    ⁡                                      (                    0                    )                                                                    j                  ⁢                                                                          ⁢                  ω                                                                    ,                            (                  Eqn          .                                          ⁢          6                )            and therefore the general solution for the flux in the motor can be characterized by:
                                                        ψ              _                        ⁡                          (              t              )                                =                                                    -                j                            ⁢                                                          ⁢                                                                                          u                      _                                        s                                    ⁡                                      (                    t                    )                                                  ω                                      +                          j              ⁢                                                          ⁢                                                                                          u                      _                                        s                                    ⁡                                      (                    0                    )                                                  ω                                                    ,                            (                  Eqn          .                                          ⁢          7                )                                                          ⁢                  =                                                                      ψ                  _                                ⁡                                  (                  t                  )                                                            Steady                ⁢                                                                  ⁢                state                                      +                                                            ψ                  _                                                  DC                  ⁢                                                                          ⁢                  transient                                            .                                                          (                  Eqn          .                                          ⁢          8                )            which yields:
The factor −j multiplying the voltage space vector ūS(t) in Eqn. 7 suggests that the steady state flux rotates with ūS(t) but lags behind in rotation by ninety degrees. The DC transient flux ψDC is, on the other hand, fixed in orientation ninety degrees ahead of the direction of the initial supply vector ūS(t) at the moment of contactor switch-on or initial voltage application, and only gradually decays. In addition, the steady state flux ψSS(t) has a constant amplitude and rotates in a manner determined by the transient ψDC which decays slowly. Hence, as ψSS(t) rotates, the presence of the DC flux ψDC causes the amplitude of the resultant flux ψ(t) to oscillate strongly. The effect is strong torque pulsations and unbalanced currents until the DC transient decays away. These starting pulsations can cause stress during motor operation and directly lead to wear, increased maintenance costs, and, ultimately, premature motor breakdown.
Torque oscillations or pulsations can be particularly problematic for a motor having its windings arranged in a delta configuration. As shown in FIG. 37, severe torque pulsation occurs with full DOL starting of a “delta” motor with simultaneous closure of the contacts for the three-phases of the motor. While this torque does decay over time, relatively strong mechanical stresses are placed on the motor from the moment of contactor closure until the decaying is complete.
The problems associated with torque oscillations are also relevant to “wye-delta” switching of delta-connected motors. Larger delta-connected motors are commonly started using a wye-delta switching method wherein the motor windings are first connected in a star or wye configuration, and the three pole contactors are closed simultaneously to supply power to all three windings. At some moment thereafter, and generally when the motor is up to normal operating speed, the contactors open and then reclose in such a manner to reconfigure the motor windings in a delta configuration. Heretofore, a timer has been used to initiate the switching from the wye to the delta configuration. Moreover, similar to the initial closure of the contactors to connect the windings in a star configuration, the contactors also re-close simultaneously when connecting the windings in the delta configuration without regard to any back force, or rotor electromotive force (emf), present in the motor as the motor slows from being disconnected from a power supply. The aim is to re-apply the supply voltages simultaneously to the three delta-connected windings with minimal delay.
The benefit of starting a motor with its windings in a wye configuration is that the motor draws only one third of the line current that it would draw if started directly with the windings in a delta configuration. This reduces adverse impact on the supply of the normally high starting current of the motor (typically 6-8 times full load current). The motor is only switched into delta configuration when sufficient time has elapsed for the motor to be running at high speed, when it draws less current. Generally, with wye-delta switching, the initial stator voltage is reduced by half and the torque and line currents are reduced by a factor of three. Nevertheless, as shown in FIG. 38, standard wye-delta switching remains susceptible, albeit to a lesser degree, to torque pulsations at both the initial wye switch-on, and at the wye-delta switch-over.
One proposed solution to reduce torque oscillations that result at the switch-over from a wye configuration to a delta configuration is to allow the back electromotive force present in the motor to decay. That is, when the motor is running with its windings in a wye configuration and the contactors are simultaneously opened, the rotor of the motor will continue to rotate. Depending upon the characteristics of the motor, the rotor will quickly or slowly expend its kinetic energy. Simply, despite no current in the stator windings, there will still be current in the rotor bars during this slow-down in rotation. The current in the rotor bars will thus induce an alternating emf that can be observed at the motor terminals. The currents decay with the rotor time constant Lr/Rr, and cause the rotor to behave as a decaying magnet that rotates at the shaft speed. Due to the slowing of the rotor under load, the back emf can be in or out of phase with the voltage supply when the contactors are re-closed in the delta configuration. If the back emf is out of phase at the moment of re-connection of the power supply to the motor windings, relatively large transient current and torque pulsations may occur. It is therefore advantageous to allow the back emf to decay before simultaneously re-closing the contactors to connect the motor windings in delta. However, as shown in FIG. 39, simply allowing the back force, or rotor emf, to decay does not eliminate torque pulsations or oscillations when the contactors are re-closed simultaneously. In FIG. 39, the emf was allowed to decay ninety percent (90%) before simultaneous re-closure of the contact to connect the motor windings in a delta configuration.
It would therefore be desirable to design a system to control power application to a motor or other power system during transitioning from one mode of operation to another mode of operation, such as from motor start-up to motor running. In this regard, negative torque oscillations and potentially damaging stresses on the motor and its components are reduced.